/emacs-ffi

FFI for Emacs

Primary LanguageC

This is an FFI for Emacs. It is based on libffi and relies on the dynamic modules work (available on the Emacs 25 branch) in order to be loaded into Emacs. It is relatively full-featured, but for the time being low-level.

I'd appreciate your feedback, either via email or issues on github.

Building

This build has only been tested on Linux. Build dependencies include

  • GCC

  • GNU Emacs which has been compiled with the --with-dynamic-modules build configuration option set.

  • libffi.

  • libltdl, also known as libtool.

  • GNU Make for building.

Of course all of Emacs, libffi and libltdl must all be built with the same GCC toolchain, but any recent build of GNU Make will do.

Use GNU Make to build

Simply change to the directory containing this README.md file and run the command:

make

You may need to specify the location of your Emacs build if you have built it locally. To do this set the EMACS_BUILDDIR environment variable, for example:

make EMACS_BULIDDIR=/home/myself/Emacs/29.1/emacs

If you have also built libffi and/or libltdl locally and installed them in /usr/local, you should also set the SYS_INCLUDEDIRS environment variable:

make EMACS_BULIDDIR=/home/myself/Emacs/29.1/emacs \
     SYS_INCLUDEDIRS=/usr/local/include

Emacs is also used to byte-code compile the ffi.el file. This is not a necessary step, but it may cause make to report an error if the emacs executable is not found in PATH.

Update you Emacs configuration file

When make completes, you will need to add this project directory to your Emacs load-path and dynamic-library-alist in your Emacs configuration file, typically ~/emacs.d/init.el. For example:

(add-to-list 'dynamic-library-alist "/home/myself/Emacs/emacs-ffi")
(add-to-list 'load-path "/home/myself/Emacs/emacs-ffi")

Defining FFI bindings in Emacs Lisp

Types

Currently the library supports primitive and structure types for arguments and return types.

Primitive types are described using keywords:

  • :void. The void type. This does not make sense as an argument type.

  • :int8, :uint8, :int16, :uint16, :int32, :uint32, :int64, :uint64: signed or unsigned integers of the indicated size.

  • :float, :double. Self-explanatory.

  • :char, :uchar, :schar, :ushort, :short, :uint, :int, :ulong, :long, :ulonglong, :longlong. Signed or unsigned integers corresponding to the C type of the same name. :char is treated specially because whether it is signed or unsigned is platform-dependent (and also command-line-argument-dependent, though normally this doesn't matter).

  • :pointer. A C pointer type. Pointers currently aren't typed, in the sense that they aren't differentiated based on what they point to.

  • :size_t, :ssize_t, :ptrdiff_t, :wchar_t. These correspond to the C type of the same name and internally are just aliases for one of the other integral types.

  • :bool. Booleans are treated in a Lisp style. As an argument type, nil is converted to a C false value, and other Lisp values are converted to true. As a return type, true is converted to t and false is converted to nil. Note that 0 is not converted to false. If you want a "numeric" boolean type, you can use the size and alignment to find the corresponding primitive type and use that instead.

Structure types are represented by a user-pointer object that wraps an ffi_type. The best way to manipulate structures is to use define-ffi-struct, which is a limited form of cl-defstruct that works on foreign objects directly.

A structure object is also represented by a user-pointer object. If a function's return type is a structure type, then the object allocated by the FFI will automatically be reclaimed by the garbage collector -- there is no need to explicitly free it. (Contrast this with the behavior of ffi-make-c-string, which requires an explicit free.)

Type Conversions

Currently all type conversions work the same in both directions.

  • A function declared with a :void return type will always return nil to Lisp.

  • A function returning any integer or character type will return a Lisp integer. Note that this may result in the value being truncated; currently there is nothing that can be done about this.

  • A C pointer will be returned as a user-pointer (a new Lisp type introduced by the dynamic module patch).

  • A structure is also represented as a user-pointer. When a structure is returned by value from a foreign function, the resulting user-pointer will have a finalizer attached that will free the memory when the user-pointer is garbage collected.

Exported Functions

  • (define-ffi-library SYMBOL NAME). Used to define a function that lazily loads a library. Like:

      (define-ffi-library libwhatever "libwhatever")

    ffi-module uses libltdl (from libtool), which will automatically supply the correct extension if none is specified, so it's generally best to leave off the .so.

  • (define-ffi-function NAME C-NAME RETURN-TYPE ARG-TYPES LIBRARY).

    A macro that defines a new Lisp function. It takes as many arguments as were in ARG-TYPES. (While there is internal support for varargs functions, it is not exposed by define-ffi-function.)

    NAME is the symbol to define. C-NAME is a string, the name of the underlying C function.

    RETURN-TYPE describes the return type of the C function and ARG-TYPES describes the argument types. ARG-TYPES may be a vector or a list.

    LIBRARY is the library where the C function should be found. This is just a symbol, most usually defined with define-ffi-library.

  • (ffi-lambda FUNCTION RETURN-TYPE ARG-TYPES). Take a C function pointer and a description of its type, and return a Lisp function. Unlike define-ffi-function, this is not a macro. You may wish to cache these as each call to ffi-lambda makes a new CIF.

  • (ffi-make-closure CIF FUNCTION). Make a C pointer to the Lisp function. This pointer can then be passed to C functions that need a pointer-to-function argument, and FUNCTION will be called with whatever arguments are passed in.

    CIF is a CIF as returned by ffi--prep-cif. It describes the function's type (as needed by C).

    This returns a C function pointer, wrapped in the usual way as a user-pointer object.

  • (ffi-get-c-string POINTER). Assume the pointer points to a C string, and return a Lisp string with those contents.

  • (ffi-make-c-string STRING). Allocate a C string with the same contents as the given Lisp string. Note that the memory allocated by this must be freed by the caller. It is done this way so that Lisp code has the option of transferring ownership of the pointer to some C code.

  • (define-ffi-struct NAME &rest SLOT...). A limited form of cl-defstruct that works on foreign objects. This defines a new foreign structure type named NAME. Each SLOT is of the form (SLOT-NAME :type TYPE). Each TYPE must be a foreign type.

    define-ffi-struct makes accessors for each slot of the form NAME-SLOT-NAME. setf works on these accessors.

  • (define-ffi-union NAME &rest SLOT...). Like define-ffi-struct, but defines a union.

  • (ffi-pointer+ POINTER NUMBER). Pointer math in Lisp.

  • (ffi-pointer-null-p POINTER). Return t if the argument is a null pointer. If the argument is not a pointer or is not null, return nil.

  • (ffi-pointer= POINTER1 POINTER2). Return t if the two pointers are equal, nil if not.

  • (ffi-allocate TYPE-OR-NUMBER). Allocate some memory. If a type is given, allocates according to the type's size. If a number is given, allocates that many bytes. The returned memory will not be automatically reclaimed; you must use ffi-free for that.

  • (ffi-free POINTER). Free some memory allocated with ffi-allocate or ffi-make-c-string.

Internal Functions

  • (ffi--dlopen STR). A simple wrapper for dlopen (actually lt_dlopen). This returns the library handle, a C pointer.

  • (ffi--dlsym STR HANDLE). A simple wrapper for dlsym (actually lt_dlsym). This finds the C symbol named STR in a library. HANDLE is a library handle, as returned by a function defined by define-ffi-library.

    This returns a C pointer to the indicated symbol, or nil if it can't be found. These pointers need not be freed.

  • (ffi--prep-cif RETURN-TYPE ARG-TYPES &optional N-FIXED-ARGS). A simple wrapper for libffi's ffi_prep_cif and ffi_prep_cif_var. RETURN-TYPE is the return type; ARG-TYPES is a vector of argument types. If given, N-FIXED-ARGS is an integer holding the number of fixed args. Its presence, even if 0, means that a varargs call is being made. This function returns a C pointer wrapped in a Lisp object; the garbage collector will handle any needed finalization.

  • (ffi--call CIF FUNCTION &rest ARG...). Make an FFI call.

    CIF is the return from ffi--prep-cif.

    FUNCTION is a C pointer to the function to call, normally from ffi--dlsym.

    ARGS are the arguments to pass to FUNCTION.

  • (ffi--mem-ref POINTER TYPE). Read memory from POINTER and convert it, using the usual conversions, as the given type. This is the Lisp equivalent of *pointer in C.

  • (ffi--mem-set POINTER TYPE VALUE). Copy the Lisp value to the memory pointed at by the pointer, using the type to guide the conversion. This is the Lisp equivalent of *pointer = value in C.

  • (ffi--type-size TYPE). Return the size of TYPE.

  • (ffi--type-alignment TYPE). Return the alignment needed by TYPE.

  • (ffi--define-struct &rest TYPE...). Define a new foreign structure type, whose fields are the indicated types.

  • (ffi--define-union &rest TYPE...). Define a new foreign union type, whose fields are the indicated types.

To-Do List

  • See the github issues.